Fluid transport 2

 

TRANSLOCATION OF ORGANIC SOLUTES

 

Young and growing structures of the plant body like shoot apex, root apex, leaf primordial, floral buds, axially buds, etc. require vitamin, hormones and adequate energy source for their active metabolism and sustained growth.  Similarly, growing fruits and other storage organs require reserve food materials for their normal development.

 

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www.scienceaid.co.uk

 

The above said materials are mostly synthesized or made available in the photosynthetic organs like leaves, phyllocades or cladodes, from where i.e. supply end, organic substances are transported to structures where it is needed.  This end is called as the sink and the supply end is called the source.  Such a process is called translocation of organic solutes.

 

www.gopixpic.comwonderwhizkids.

 

 

 Sieve tubes are living cells which contain cytoplasm but do not have nucleus. So its function is supported by companion cell. Each sieve tube has a companion cell next to it. Food is prepared in the Mesophyll cells of a leaf. Food is translocated in the form of sucrose first into companion cells leaf veins in the leaves. The movement of water and dissolved minerals in xylem is always upward (from soil to leaves and stem apexes). The movement of food can be upward as well as downward depending upon the needs of the plants.

 

Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of K+ by root cells. In the mechanism called co-transport, a transport protein couples the downhill passage of one solute (H+) to the uphill passage of another (NO3). The "coat-tail" effect of co-transport is also responsible for the uptake of the sugar sucrose by plant cells. A membrane protein co-transports sucrose with the H+ that is moving down its gradient through the protein. http://wonderwhizkids.com/

 

 

 

Determining the structures involved in Translocation

 

Ringing Experiment: To find out whether, the xylem or phloem tissue is involved in translocation, it is possible to remove the cortex and phloem of the stem in the form of a girdle.  If the xylem is involved in the transportation, the roots found below the ring should not suffer because the xylem is intact in this experiment.  But in this experiment, roots suffer temporarily because; they don’t get adequate supply of food material, for phloem and cortex is removed.  So the food material accumulates at the edge of the ring and with time this region swells into a ridge.  This experiment suggests that phloem elements are involved in the translocation of food materials.

 

Radioactive tracer experiment

 

If radioactive traces like 14CO2 or (14C) sucrose is fed to leaves and chased, i.e. followed with time, it is possible to follow the pathway through which the photosynthate or sucrose is moving.

Organic solute transport to sink such as tubers in plants; www.uic.edu

 

www.uic.edu; teacherweb.com

 

 

 

                        Radioactive tracers used to locate solutes transfer; www.saylor.org

 

Tracer studies with autobiography or sap analysis clearly show that the sieve tubes are the structures through which organic materials are translocated.  This method can be used to determine even the rate of translocation.

 

 

Experiments with aphids provide the information of the contents sieve tube translocates.

 

Structure and organization of sieve elements:

 

Among the different phloem elements sieve tube cell and companion cell form an inseparable pair of cells.  And sieve tube acts as the main structure which translocates the organic solutes. 

Anatomical structures involved in Translocation of solutes:

 

 

 

Sieve tubes associated with companion cellswww.cell.com

www.driverlayer.com

 

Though both cell types are derived from the same procambial cells, sieve cell undergoes considerable expansion in length and breadth.  In order to keep pace with the growth of the sieve cell, the companion cells divides once, or twice for every single sieve tube that develops. Due to expansion of the sieve cells, a large central vacuole develops with thin peripheral protoplasm.

 

The structure of sieve tubes, which comprise phloem

https://online.science.psu.edu

 

https://online.science.psu.edu; www.gopixpic.com

Phloem loading: To establish a high pressure potential in sieve tube members near source cells, large amounts of sugar have to be transported into the phloem sap-enough to raise the solute concentration of sieve tube members. -Phloem loading is thought to depend on a proton pump, (H+-ATPase) and a cotransporter, a protein that serves as a conduit for protons and sucrose to enter the cell together. 
-Uses energy (ATP)


Phloem unloading: Occurs when cells take up sucrose.
-It depends on the sink of where the sucrose is leading too.
-Energy use varies between tissues: energy required in storage tissues but not in growing tissues

 

www.cnx.org; http://www.uic.edu

 

 

www.cell.com23

The areas of the plant where sucrose is ...

http://igbiologyy.blogspot.com

 

 

Sieve element associated with Transfer cell interconnected. biochemunrated.wordpress.com

 

 

It is during the enlargement, the nuclear membrane disappears and the chromatin material breaks up and gets distributed through out the protoplasm. Almost at this juncture a massive amount of mRNAs that are transcribed by the chromatin bits left free, perhaps stored to used when required.  In the case of RBC mRNA remain active for more than 120 days and human erythrocytes lack nuclei similar to sieve cells.  The proteins that are synthesized due to such transcription and translational activity are none other than tubulins, which soon organize into longitudinally oriented microtubules.  The microtubules are also associated with microfilaments and smaller microtrabcular structures.  Such Protein complex is called phloem proteins.  The longitudinally oriented parallel bundles of microtubules and microtrabacular filaments actually traverse across the transverse septa called sieve plate which is highly perforated. 

Sucrose and monosaccharide transporters mediate long distance transport of sugar from source to sink organs and constitute key components for carbon partitioning at the whole plant level and in interactions with fungi. Even if numerous families of plant sugar transporters are defined; efflux capacities, subcellular localization and association to membrane rafts have only been recently reported. On the fungal side, the investigation of sugar transport mechanisms in mutualistic and pathogenic interactions is now emerging. Here, we review the essential role of sugar transporters for distribution of carbohydrates inside plant cells, as well as for plant–fungal interaction functioning. Altogether these data highlight the need for a better comprehension of the mechanisms underlying sugar exchanges between fungi and their host plants. Sugar Transport in plants- Joan Dokly et al. Trands in Plant scince; http://www.cell.com/

 

Sugars from mesophyll cells are translocated into phloem companion cells. This creates reduced water potential, thus water from xylem enters into companion cells/sieve tubes thus creates pressure potential, which pushes the organic solutes towards the lower potential such as root, tubers and stem apex; The question bidirectional movement of organic solute in the same sieve tube is questionable.  If this has be explained, one has to visualized as having transport tracts where the food is carried as found in other systems, where microtubules are involved in cargo transport, but no such organized microtubules or actin fibers are found in sieve tubes, or P-proteins involved in the transport is not known.www.plantphys.info

 

Comparison of source-to-sink sugar transport in symplastic and apoplastic active phloem loaders. Sucrose available for export from mesophyll cells (MC) results from a balance between storage in the vacuoles and sequestration as starch in the chloroplasts. Sucrose can reach the sieve tubes through plasmodesmata that allow for its diffusion from cell to cell in species like cucurbits. Sucrose is converted to larger molecules (RFOs) by the sequential addition of galactosyl residues in modified companion cells (CC) called intermediary cells. The larger molecules cannot move back to phloem parenchyma cells (PP) and are transferred and accumulated in sieve tubes. In apoplast-loading species, sucrose reaches phloem parenchyma cells through plasmodesmata. Sucrose is loaded and accumulates in the phloem by passing through the apoplast between the PP and the CC. The major players are presented in the enlargement of that area. Sucrose enters the apoplast through facilitators of the SWEET family (pale green circle) and is accumulated in the companion cell by a proton/sucrose co-transporter of the SUT1/SUC2 type (green circle). The energy necessary for the co-transport is provided by an H+/pumping ATPase (black circle) which establishes a proton gradient and a trans-membrane potential regulated by potassium channels of the AKT2/3 type (white circle). In Solanaceous species, SUT1 transporters are localized at the plasma membrane of sieve elements (not shown). Polyols can also be transported into the phloem, with specific transporters located in the plasma membrane of CC (not shown). A high hydrostatic pressure is generated in the sieve tubes of the collection phloem and water from the xylem is attracted. Sucrose, RFOs and polyols are transported in the sieve tubes to the sink organs in the transport phloem. All along the path, they can be leaked from and reloaded into the phloem via the same mechanism (not shown). Sucrose is unloaded into the release phloem where the hydrostatic pressure is supposed to be lower. Sucrose can be unloaded through a symplastic pathway or through an apoplastic pathway. In the latter case, sucrose is unloaded into the apoplast through specific carriers which can be of the SUT1/SUC2 type (green circle; Carpaneto et al., 2005). Sucrose is then taken up by sink-specific sucrose carriers of the same SUT1/SUC2 (light green circle) or converted to hexoses by a cell-wall invertase (CWInv). Hexoses are then taken up by specific carriers at the plasma membrane (orange circle) or at the tonoplast level (yellow and brown circles). Sucrose in sink cells can be metabolized (growing sinks) or stored as starch in amyloplasts, or imported into the vacuoles (red circles) and further converted to hexoses by a vacuolar invertase (VInv). Remi Lemoine and fourteen others http://journal.frontiersin.org/

Model of the plant’s responses to mineral nutrient deficiency. (A) Response to nitrate and phosphorus deficiency: deficiency in nitrogen and phosphorus leads to reduced photosynthesis, accumulation of sugars in source leaves, increased carbon allocation to the roots and a higher root/shoot ratio. Moreover, phosphorus limitation induces an adaptation of the root system architecture: root hairs initiate and elongate, which increases the root surface area. AtSUC2 (green circle) is a component of the sugar-signaling pathway in the response to phosphorus starvation.(B) Response to magnesium and potassium deficiency: Mg deficiency increases the concentration of soluble sugars and starch in leaves and reduces leaf growth. Mg deficiency impacts sugar metabolism, as well as sucrose export to the roots. Mg deficiency reduces the Mg-ATP availability and the activity of H+-ATPase, thus reducing the driving force for sucrose phloem loading. AKT2/3 potassium channels affect sugar loading and long-distance transport by regulating the H+/sucrose transporter. Conversely, K+-limitation rarely results in starch accumulation. MC, mesophyll cell; CC, companion cell; PP, parenchyma phloem; MC, mesophyll cell. http://journal.frontiersin.org/

 

 

Thus the proteins provide a kind of continuum between the longitudinally aligned sieve tube cells. Once the P. protein complex develops the distinction between the peripheral cytoplasm and the central vacuole disappears.  Interestingly, the mRNAs produced in sieve tubes have longer life period of 2-5 months.  Besides the above said components, sieve cells also possess slime bodies, mitochondria and proplastids and traces of endoplasmic reticulum.

 

The transverse septa found between two adjacent sieve cells are highly perforated and callose is deposited on either side of the septa in the form of cushions.  Through each of these pores, more than 100-150 parallel array of P. proteins pass through, thus the cellular components pass through from one cell to the other.  Actually recent investigations suggest that the transcellular strands that traverse the sieve plates are made up of plasmodesmata, an extension of ERs loaded with actin and its associated motor proteins such as Myosins.

 

Companion cells, in spite of deriving from the same precursor cell, does not undergo such transformation as in the case of sieve cells, but it undergoes one or two transverse mitotic divisions and the cells retain all its cytoplasm without any vacuolar formation.  More than that companion cells contain a large number of active mitochondria.  The lateral wall between the companion cell and sieve cell is perforated through which a large number of plasmodesmata pass through, so that substances can pass through from one cell to another.  Thus companion cells provide the necessary energy for the efficient function of the sieve tube.

 

Sieve tubes are arranged longitudinally one above the other so as to form a continuous column, which runs from one end of the plant to the other end.  At the nodal regions, among the sieve tubes which emerge from the petiole, some run upwards in the stem and some downwards i.e. towards the root.  Thus sieve tubes provide a specialized network of longitudinally oriented living micro pipelines for the transportation of food materials from the leaves to other regions of the plant body.  Notably, the presence of phloem parenchymatous cells around sieve tube it is very interesting, because they are in contact with both sieve tubes on one side and mesophyll cells at the other end of the leaves.  Structurally and functionally they look like transfer cells.  Further more, water containing xylem elements are also in close proximity with the sieve cells.

 

Sieve tubes are now known to conduct signal transductions generated by leaves in response to Photoperiodism.  Some of the signal molecules produced in leaves, especially mesophyll cells are translocated to sieve tubes, it is through them the signal molecules are transported to shoot meristems, where the vegetative meristems get transformed into floral meristem.  It is now known that light induced “florigin” an elusive flowering substance is none other than a protein called FT (floral locus T).  Sieve tubes are the channels through which these molecules are transported; now it is an established fact.

 

 

High-pressure manifold model that describes phloem transport from source to sink and partitioning of resources (water and dissolved solutes) between sinks. Sucrose is loaded (brown arrows) into the collection phloem (minor veins; yellow borders) of source leaves to high concentrations (dark purple) that drives an osmotic uptake of water (blue arrows). Walls of the collection phloem SEs resist the volume change with a consequent development of high hydrostatic pressures (example given, 1.4 MPa and see Table 1). STs form conduits interconnecting sources (dark green) to sinks (light purple) in a supercellular symplasm comprising collection, transport (dark green) and release (khaki) phloem. Hydrostatic pressures, generated in collection phloem SEs, are rapidly transmitted throughout the entire ST system and maintained by pressure-dependent retrieval of leaked sucrose and hence water (curved brown and blue arrows respectively). Thus STs are conceived to function as high-pressure conduits rendering resources equally available throughout a plant. Transported resources are unloaded from the release phloem SE/CC complexes as a bulk flow through high resistance (low conductance Loand see Equation 2) plasmodesmal pathways into the sink cells. SE/CC unloading imposes the greatest constraint over resource flow through the source-transport-sink pathway. As a consequence, resource partitioning between sinks is finely regulated by their relative hydraulic conductances of plasmodesmata linking SE/CC complexes with the surrounding phloem parenchyma cells. http://www.frontiersin.org/

 

Turgor homeostasis model of phloem unloading in developing seeds of grain legumes. Sucrose uptake by, and storage, in cotyledons is coupled through a negative feedback transcriptional regulation of sucrose transporter activity mediated through intracellular sucrose levels. Activities of seed coat sucrose effluxers are coordinated with cotyledon demand by a turgor-homeostat mechanism that osmotically (π) detects alterations (error signal) in apoplasmic sucrose pool sizes as a deviation from a turgor (P) set point. Symplasmic unloading from release phloem SE/CC complexes by bulk flow is regulated by plasmodesmal hydraulic conductances (Lo) under control of the turgor homeostat functioning as a central control hub to regulate resource flow from SE/CC complexes to ultimate storage in cotyledons.

 

Rate and Magnitude of translocation:

 

The rate of translocation of organic food materials can be determined by providing a pulse of 14C sucrose or 32 P-ortho phosphates to leaves and then chases it.  Results indicate that the rate of translocation ranges from 55 cm to 290 cm/hr. which again depends upon the season, the time of the day, temperature and other factors.  In spite of the presence of a large number of sieve plates across, there are 1000-2000 sieve plates for every 1 cm length of the stem; the rate of translocation is 500-1000 times faster than the rate of simple diffusion.  However, not all components show the same rate of translocation.  For example, carbon compounds like sucrose, glucose and fructose move faster than phosphates and tritiated water.  Different amino acids exhibit different rates of movement within the same plant body.

 

Use of radioactive isotopes has come very handy in obtaining accurate qualitative and quantitative data.  This can be calculated based on the amount of radioactive material fed at the supply end i.e. Leaves and the amount of radioactive material received at the receiving ends like stem apex, root apex, fruits and tubers.  This way it has been estimated that the amount of organic carbon source translocated ranges from 200 mg to 800 mg per hour.  Particularly plants like Cocos nucifera, Borasus, Phoenix and such palm plants transport enormous amount of sap ranging from 20 ml to 200 ml per hour.  More than 18% of the sap in them is sugar.

 

CHEMICAL COMPOSITION OF TRANSLOCATE

 

Aphid stylets are the best to obtain the cell sap from the sieve tubes. Once the insects are allowed to bite into the stem, they pierce their stylets into the stem so as the ends of sty lets reach the sieve tubes exactly.  Then the insects can be etherized and heads can be severed, so as to allow the sap to exudates out of the free ends of the stylets.  Chemical analysis of such sieve tube exudates reveal the presence of various organic compounds like sucrose, glucose, fructose, almost all amino acids, enzymes, vitamins, phytohormones, sugar alcohols, oligo saccharides like stachyose, raffinose and RNA etc.

 

Recent work shows leaves produce FT transcripts, under the influence light period, which are transported to leaf companion cell.  From there the FT mRNA with its bound proteins are transported to apical meristems where they act to produce the required components of inducing flower. But more than 80% of the total translocates is sucrose.  However, the glucose and fructose found in the sap are rather hydrolytic products of sucrose than the actual translocates from the source.

 

Phloem exudates have been analyzed on 2-D acrylamide gel and the same is shown below.

Two dimensional polyacrylamide gel electrophoresis separation of proteins in phloem exudate from Lupinus albus.  The gel was developed in the first dimension by isoelectric focusing with a linear pH gradient of 3-10 followed by separation due to differences in molecular mass.  The positions of mass standards are shown on the right hand side of the gel.  After staining with Coomassie Blue to locate the spots they were excised for digestion with trypsin. The peptides were then analysed by partial sequence determination using MS/MS and identified using database searches.  (Courtesy of Craig Atkins); http://plantsinaction.science.uq.edu.au/

 

DIRECTION OF TRANSLOCATION

 

Organic solutes are translocated from the region of synthesis or storage to the region of need like shoot and root apexes, developing leaves, flowers and fruits.  It is interesting to determine the direction of the translocates orginating from a leaf.  If radioactive (14C) sucrose is supplied to leaves situated at different positions on the stem, it is possible to follow the direction of the radioactive sucrose precisely.  Such experiments reveal that the leaves situated near the shoot apex translocate most of their organic solutes towards shoot apex.  Similarly, leaves located at the base of the stem translocate food towards the root.  But the leaves found in between them, translocate upwards as well as downwards.  However the bidirectional movement changes with time as more and more leaves are added to the young growing stem and its branches.  But the bidirectional movement posses certain problems like, whether the bidirectional flow takes place in different sieve tube assemblies or in the same sieve tube.  Some physiologists assumed that as sieve tube is living, it exhibits bidirectional protoplasmic streaming in the same cell, but this view has no supporting evidences.  Nonetheless, it is now accepted that the bidirectional movement is due to the transport of solutes in different sieve tube assemblies which are organized into parallel arrays. But recent investigations into microtubule and actin-myosin mediated transport demonstrates that two different microtubular and actin-myosin complex units can transport materials in two different directions.  However one complex shows one directional flow.   Interestingly, in germinating seeds or plantlets sprouting from tubers, it is observed that the food material always moves from the source to the terminal regions of the plant body where growing structures are located.

 

Though most of the food material is translocated vertically through the longitudinally oriented sieve tubes, it is not hard to imagine the lateral or radial transportation of solutes into the cells found in the cortical cells found around the phloem elements.  This lateral diffusion is facilitated by the fact that they are in contact with sieve elements.  More than that, the lateral diffusion is absolutely essential for their normal metabolic activities and survival.

 

Factors that control translocation:

 

Light and Darkness:  Light has many effects on biological systems of which photosynthesis is very important.   Translocation of food in plants is also influenced by the presence or absence of light.  If radioactive 14C sucrose of known quantity is fed to the leaves of two different plants of the same age and then one plant is kept in dark and the other in light either for short duration or longer duration the recovery of radio active isotopes shows greatest amounts of radioactivity n the roots, of the plant that is kept in dark, than the plant kept in the light.  Though there is a general tendency for the movement of more food material towards roots than shoots, light favors greater movement of organic solutes towards shoot apex and darkness favors the opposite.

 

The action spectrum of the light that favors translocation has been found to be red and blue which are acts as the action spectrum for photosynthetic reactions.  Interestingly, most of the glucose synthesized by leaves during the day is stored as starch and the same is converted into sucrose at nights and transported out of the leaves.  This observation can be tested by keeping the plant in dark for 24-48 hours.  Afterwards, if leaves are tested for starch with KI, they do not show any coloration.  But the leaves of the plants exposed to light from morning to evening show maximum deep blue color for KI test than the leaves exposed to light for one  or two hours.  The above observations clearly suggest that during night plants translocate most of their food materials from the leaves to other regions, but during day time they do translocate some food materials, but most of it is stored in the form of starch.

 

Temperature:   Translocation is greatly affected by the change in the temperature, which means, the process is a facilitated process and it needs metabolic energy.  If the petiole through which organic materials are translocated is cooled by water jackets, translocation is greatly affected.  Similarly, if roots or stem tips are subjected to cold temperatures, the translocation of solutes towards them is severely inhibited.  On he contrary, if the temperature is increased, the rate of translocation also increases.  However very high temperature is fatal,  but, certain cold resisting plants like sugar beet are capable of adapting to cold conditions by producing antifreeze proteins and maintain the normal flow of food materials even at freezing temperatures.

 

Effect of Phytohormones:   Growth promoting hormones like indole acetic acid Gebberllins and cytokinins are found to accelerate the rate of translocation.  As the above said hormones activate cellular metabolic activities, cell division and cell elongation in the apexes and other growing tissues, the said structures need greater amount of organic solutes energy and structural components.  Hence these regions act as sink and organic solutes move in to them with greater facility.  Though individual hormones have different but specific effects on plant growth, they accelerate the rate of movement of materials from the source to the sink.  For example, IAA increases the rate of movement from the leaves to other regions.  Cytokinin favors the translocation of sucrose out of the leaves, but preserves nitrogenous compounds within the leaves.  On the other hand, if the same hormone is applied to the roots, it favors as well as accelerates the movement of sucrose, amino acids and other compounds towards roots.

 

To substantiate the favorable effects of phytohormones on translocation, if an agar block containing IAA or GA is placed over the decapitated stem and 14C sucrose or 14CO2 is fed to the leaves, the sucrose or the photosynthate found in leaves moves towards the region at which hormones is supplied.  Phytohormones, have stronger influence not only on the direction of translocation, they also accelerate the rate of translocation.

 

Effect of minerals:  Among all the essential nutrients, boron has been found to facilitate the movement of sucrose.  The absence of boron reduces the rate of translocation significantly.  Whether boron forms an ionizable complex with sucrose or it facilitates the formation of more of sucrose for transport is not clear.  Phosphate has also been implicated in this process but not much is known, how phosphate facilitates or affects the process of translocation.  Even K+ ions and other metallic ions have thought to play an important role in electro-osmosis, but their exact mechanism is not substantiated by any reasonable experiments.

 

Effect of Concentration Gradient:

 

Translocation between the supply end and the receiving end is always governed by the concentration gradient.  Steeper the gradient, greater is the rate of translocation and vice versa.  Such a gradient mediated movement of solutes can be demonstrated by cutting off the leaf which acts as the supply end.  By removing the leaves, the gradient will be abolished and automatically the transportation comes to stand still. Generally at the supply end, sieve tubes contents show greater positive pressure and the receiving ends like stem apex, root apex, developing flower or fruit show negative pressure.  This provides a motive force for the rapid movement of organic solutes from leaves to the other said regions called sinks.

 

MECHANISM OF TRANSLOCATION

 

Various theories have been proposed from time to time, but there is not even a single theory which can explain the phenomenon completely and explicitly.  Theories like ‘active diffusion’, protoplasmic streaming proposed by Devries and Curtis, electro osmosis by Spanner and Jones, transcellular streaming by Thiane and Ganny, mass or pressure flow by Munch are based on certain observations which are incomplete in their explanations.  Sometimes the assumptions are misguided.  Still most of the above said theories have greatly contributed in understanding the mechanism of translocation, because they have explained one or the other factual observations.  But in recent years, the understanding of sieve cell at the ultra structural level, and the use of radioactive tracers, have helped in reconsidering and remodeling the original individual concepts into an unified mechanism, for the entire process is not operated by a single force or a single event, it is a combination of many forces which act as a unified mechanism which operate at different levels or steps; such as vein loading, development of mass pressure and active transportation, etc.

 

VEIN LOADING

 

At nights starch found in plastids are first degraded into glucose and 50% of it is converted to fructose 6P.  Even during day periods, certain amount of glucose produced by photosynthesis is converted to Fructose 6P.  The required glucoses are activated by uridine diphosphate to uridine diphosphate glucoses.  Then UDPG combines with fructose phosphate to produce sucrose phosphate.

 

The enzyme responsible for this process is sucrose synthetase.  This process takes place within the plastids.

 

Starch                          -           Glucose

Glucose                                   -Glucose 6P

Glucose 6P                              -Fructose 6P

Glucose 6P+U              -           UDPG+2Pi

UDPG + Fructose 6P-->>         Sucrose-P + UDP

 

 

Sucrose

  Mechanisms of solute transport. Three distinct types of passive transport, as well as active transport, are illustrated. Passive transport is the movement of solute across a membrane down an electrochemical gradient (from the side of the membrane with a high concentration of solute to the side with a low concentration). Active transport is the movement of solutes across a membrane against an electrochemical gradient (from the side of the membrane with a low concentration of solute to the side with a high concentration), which requires energy. Simple diffusion is the unassisted diffusion of a lipid-soluble solute (shown as blue dots) across a lipid bilayer. Transport via an ion channel involves a protein-lined pore (shown in purple) spanning the lipid bilayer through which select ions (shown as green dots) are transported. Facilitated diffusion involves transport of a solute (shown as green triangles) across a membrane by a carrier protein (shown in purple). Active transport involves transport of a solute (shown as green squares) against an electrochemical gradient by a pump protein (shown in purple). This process requires energy. https://wikispaces.psu.edu

 

The H+/Sucrosepump; www.uic.edu

Four different mechanisms of active transport. Active transport is defined as the transport of solute against an electrochemical gradient, so energy is required. The ATP-dependent pump uses the energy derived from ATP hydrolysis to ADP and inorganic phosphate (Pi) to drive the transport of a solute (green circle) against its electrochemical gradient. The light-driven pump uses the energy of absorbed photons of light to transport a solute (blue circle) against its electrochemical gradient. The symporter transports two different solutes (orange circle and square) in the same direction across a membrane. One of the solutes (e.g. the orange circle) is transported against its electrochemical gradient, hence this is active transport. This transport requires the free energy released from the movement of a second solute (e.g. the orange square) down its electrochemical gradient. The antiporter transports two solutes (purple circle and square) in opposite directions across a membrane. The transport of one solute is against its electrochemical gradient (e.g. the purple circle). This is driven by the coupled transport of a second solute (e.g. the purple square) in the opposite direction, down its electrochemical gradient; https://wikispaces.psu.edu

 

 

Sucrose P thus produced is released into cytoplasm.  The specific carriers pickup the sucrose molecules and transport the same from cell to cell towards vascular bundles.  The protoplasmic strands found as inter cellular connecting strands are very active and help in the transportation of materials from one cell to another.  Finally sucrose is transported into vascular bundles, where parenchymatous cells associated with sieve tubes greatly help in loading the sucrose into relatively empty sieve tube cells.  Interestingly, in the mesophyll tissues where the veins end or begin the sieve tubes are invariably kept empty so as to draw photosynthates towards the main translocation stream. So the process of loading of veins goes on and on, however vein loading is more active during night than at daytime.

 

 

DEVELOPMENT OF MASS PRESSURE

 

As several hundreds and thousands of sieve tubes, located at the vein ends, are loaded with sucrose, a steep DPD gradient develops between the sucrose laden sieve tube cells and water containing xylem elements.  Automatically, water is drawn rapidly into sieve cells enmass. 

 

www.getdomainvids.com

 

 

www.getdomainvids.com

 

 Munich's "Pressure Flow" model (1927).

 

Munich's model best describes the movement in the phloem. His model suggests that there is a turgor-pressure gradient that drives the directional mass flow of the solutes and water through sieve tubes of the phloem.

 

http://www2.mcdaniel.edu/

 

 This can be demonstrated with an osmometer permeable only to water filled with a high concentration of solutes in one arm as shown below. When the osmometer is put in distilled water, the water potential is less than that of surrounding water in the one arm, and water will enter by osmosis which the generates a turgor pressure, The solutes are carried by bulk flow when the water moves in. Eventually this process will stop when the pressure throughout equalizes. http://www2.mcdaniel.edu/

 

 

Using this analogy, phloem transport can be explained as follows:

1. Sucrose is loaded into sieve tubes by active transport. 2. Water potential decreases and water enters sieve tubes by osmosis, turgor pressure. 3. Turgor pressure pushes solutes to sink, were it is needed while water moving in and out of the sieve tubes. 4. Removal of sucrose at sink increases water potential causing water to move out of the sieve tube at the sink. 5. Solutes move to sink cells and water goes back to xylem.

ATP and H+-carrier in the cell membrane are used to pump protons out of the sieve tube.

2. A proton gradient forms across the membrane with high H+ concentration on the outside of the membrane, K+ enters to keep charge balance.

3. Diffusion of proton back into the sieve tube, through ATP-ase, is coupled to a carrier and powers the transport of sugar into the sieve tube.

http://www2.mcdaniel.edu/

In this model, there is both passive and active transport. Passive: Long distance. Osmosis, water movement into and pushing solutes down the tube. Active: Short distance movement, loading and unloading the sugar.

 

ATP is important in this process because: - It increases Phloem loading; - Sugars are pumped into sieve tube against a concentration gradient: the concentration of sucrose in the chlorenchyma cell is typically 10-50 mM, while that in a sieve tube of a minor vein of a leaf may be as high as 1M.

This process of phloem loading increases pH in the sieve tubes to 8 and decreases pH out to 5.5, that is why K+ is used to maintain a balance.

Unloading: Unloading may occur symplastically or apoplastically. It may vary in different species.

Loading: Loading of sieve tubes from the cell walls requires energy which is derived indirectly by the proton gradient.

1. ATP and H+-carrier in the cell membrane are used to pump protons out of the sieve tube.

2. A proton gradient forms across the membrane with high H+ concentration on the outside of the membrane, K+ enters to keep charge balance.

3. Diffusion of proton back into the sieve tube, through ATP-ase, is coupled to a carrier and powers the transport of sugar into the sieve tube.

ATP is important in this process because: - It increases Phloem loading; - Sugars are pumped into sieve tube against a concentration gradient: the concentration of sucrose in the chlorenchyma cell is typically 10-50 mM, while that in a sieve tube of a minor vein of a leaf may be as high as 1M.

This process of phloem loading increases pH in the sieve tubes to 8 and decreases pH out to 5.5, that is why K+ is used to maintain a balance.

Unloading: Unloading may occur symplastically or apoplastically. It may varyin different species. http://www2.mcdaniel.edu/

Schematic diagram of transfer and transport processes contributing to the flow of assimilates acquired from aerial or soil environments, through the source-path-sink system. CO2 fixed by photosynthesis in chloroplasts gives rise to sucrose and starch. Sucrose, amino acids and mineral nutrients are loaded into sieve element—companion cell (se—cc) complexes of leaf phloem for long-distance transport to non-photosynthetic sinks. These solutes are exchanged reversibly between se-cc complexes and short- and long-term storage pools along the axial pathway. Short-term storage pools include phloem apoplasm, whereas the protoplasm of non-transport cells provides a long-term storage pool. In sink tissues, solutes are used for respiration, growth or storage.www.plantsinaction.science.uq.edu.au

 

As a result, massive hydrostatic pressure develops within the sieve tubes at the loading end, while the receiving end of the sieve tube micro pipeline assembly is kept under negative pressure because the organic solutes are constantly utilized by the active and expanding cells.  Thus, a steep gradient develops between the supply end and the receiving end.  This acts as the motive fore, where massive hydrostatic pressure developed at the supply end pushes the sucrose and other accompanied organic components enmass towards the sink.  Munch demonstrated this phenomenon by an experiment.

 

ONWARD MOVEMENT BY ACTIVE TRANSLOCATION

 

Once sieve tubes are subjected to massive hydrostatic pressures, the sucrose and other components that are loaded into sieve cells are virtually pushed forward along the concentration gradient.  The initial motive force in this process is the hydrostatic pressure.

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The basic idea of how flow in the phloem works was described by Ernst Münch in 1930. The concept is a pressure-driven bulk flow model. To understand this you have to realize that materials flowing from sieve tube element to sieve tube element do not go through any membranes…so the phloem is basically one cell, continuous from leaf to root and leaf to apical bud. A basic diagram here helps us to understand how this works:

As you can see, the cells of the leaf produce sugars that are loaded by transfer and/or companion cells into the phloem. The resulting reduced Ψs here lowers the water potential and water moves from the xylem into the phloem cell too. The combination of sugar and water moving into the phloem cell increases the pressure Ψp in the cell, pushing cytoplasm by bulk flow away from the leaf and toward the sink organs (root, etc.).

A pressure gradient is established along the phloem column from leaf to sink. The pressure is reduced along its length as sugar is taken out by adjacent cells and water follows osmotically. In the ultimate sink tissue, there is still sugar present to be unloaded and for water to follow. Thus, from source to sink, the pressure gradient is what provides the power for bulk flow. It is subject more to Poiseuille’s than to Fick’s factors.

Because the source is in the middle of the plant, with sinks at each end of the plant, the idea of phloem as a huge symplast with unloading at each end, and loading in the source area, explains the required bidirectional flow of solutes translocated in phloem. http://horticulturetalk.wordpress.com/

 

 

 

Comparison of source-to-sink sugar transport in symplastic and apoplastic active phloem loaders. Sucrose available for export from mesophyll cells (MC) results from a balance between storage in the vacuoles and sequestration as starch in the chloroplasts. Sucrose can reach the sieve tubes through plasmodesmata that allow for its diffusion from cell to cell in species like cucurbits. Sucrose is converted to larger molecules (RFOs) by the sequential addition of galactosyl residues in modified companion cells (CC) called intermediary cells. The larger molecules cannot move back to phloem parenchyma cells (PP) and are transferred and accumulated in sieve tubes. In apoplast-loading species, sucrose reaches phloem parenchyma cells through plasmodesmata. Sucrose is loaded and accumulates in the phloem by passing through the apoplast between the PP and the CC. The major players are presented in the enlargement of that area. Sucrose enters the apoplast through facilitators of the SWEET family (pale green circle) and is accumulated in the companion cell by a proton/sucrose co-transporter of the SUT1/SUC2 type (green circle). The energy necessary for the co-transport is provided by an H+/pumping ATPase (black circle) which establishes a proton gradient and a trans-membrane potential regulated by potassium channels of the AKT2/3 type (white circle). In Solanaceous species, SUT1 transporters are localized at the plasma membrane of sieve elements (not shown). Polyols can also be transported into the phloem, with specific transporters located in the plasma membrane of CC (not shown). A high hydrostatic pressure is generated in the sieve tubes of the collection phloem and water from the xylem is attracted. Sucrose, RFOs and polyols are transported in the sieve tubes to the sink organs in the transport phloem. All along the path, they can be leaked from and reloaded into the phloem via the same mechanism (not shown). Sucrose is unloaded into the release phloem where the hydrostatic pressure is supposed to be lower. Sucrose can be unloaded through a symplastic pathway or through an apoplastic pathway. In the latter case, sucrose is unloaded into the apoplast through specific carriers which can be of the SUT1/SUC2 type (green circle; Carpaneto et al., 2005). Sucrose is then taken up by sink-specific sucrose carriers of the same SUT1/SUC2 (light green circle) or converted to hexoses by a cell-wall invertase (CWInv). Hexoses are then taken up by specific carriers at the plasma membrane (orange circle) or at the tonoplast level (yellow and brown circles). Sucrose in sink cells can be metabolized (growing sinks) or stored as starch in amyloplasts, or imported into the vacuoles (red circles) and further converted to hexoses by a vacuolar invertase (VInv). http://journal.frontiersin.org/

 

 

In spite of the development of mass hydrostatic pressure at the supply end, it does not explain the rate of diffusion of organic solutes in such massive quantities, because the rate of translocation of solutes reaches an astounding rate of 55-5000 cm/hr; which is actually 10,000 to 20,000 times faster than the recorded rates of diffusion, furthermore, the translocation is an active process that is ATP dependent. 

 

 

 

This impasse has been explained by involving P-proteins which run from one sieve cell to another as inter connecting active tubular system.  The P Proteins consisting microtubules, microtrabaculae and actin-myosin elements actively transport organic solutes, preferably sucrose along the surface of microtubular elements.  In fact, the organic solutes are virtually pushed or glided over the surfaces of tubular microtubules by the activity of kinesin like protein activity.  The process is active because it expends ATP energy.  The required energy is supplied by mitochondria that are present in sieve tube cells per se; but most of the energy rich molecules are supplied by associated companion cells.  Furthermore, the flow of organic solutes all along the surface of phloem microtubular elements is so smooth, it is never impeded by the presence of sieve plates for the tubules are continuous and they run from one end to the other.  In fact, such activated transport can reach a velocity of 55-500 cm/hr which is actually an observed fact.

 

EVIDENCES TO SUPPORT THE ABOVE SAID MECHANISM

 

The above said events as a mechanism of transportation of organic solutes are supported by the following facts:

 

1.      The presence of longitudinally oriented P-protein having microtubules, microfilaments and microtrabaculae has been demonstrated by high voltage electron microscopy. Such protein complexes are found across the sieve tubes running from one sieve tube to the other.

 

2.      In vitro polymerization of tubulin monomers into tubular microtubules has been perfecting.  Using invitro microtubules, it has been shown that micro tubules spread on a glass plates perform the movement of partientated on their outer surfaces.

 

3.      The mass pressure developed in sieve tubes present in leaves shows greater positive pressures.  In fact, the estimation of sucrose content of sieve tubes at the vein loading site is about 20-40% which is equivalent of + 20-40 atmospheric pressure.

 

4.      The entire process is energy dependent, for the respiratory inhibitors like KEN, DNP, Rotenone, etc; readily inhibit the active transportation of organic compounds to a greater extent.  Still the diffusion of organic solutes continues along with the concentration gradient, which when compared to the normal process is many thousand times slower.  Use of radioactive tracer elements like 35 S-Methionine, has shown that though the sieve tube cells are enucleated, the cells are active in the synthesis and polymerization of P-Proteins and its associated components.  Such proteins exhibit rapid turnover during the transportation of organic solutes.

 

5.      Colchicine treatment completely abolishes the transport of sucrose similarly most of the sieve tubes under cold conditions prevent polymerization of tubulin into microtubules, so hinder the movement.

 

PROTOPLASMIC THEORY

 

Protoplasmic theory is based on the observation of protoplasmic streaming in other plant cells, where the protoplasm exhibits rapid circulatory movements in which it carries some visible small granular structures.  As sieve tubes being living cells, DeVries has assumed that the solutes are transported along the protoplasmic stream from one cell to the other across the sieve plates.  If the observed rate of protoplasmic streaming is compared to the actual rate of translocation, it is slower at least by 1000 times.  This process does not implicate the requirement of mass flow or pressure flow.

 

TRANSCELLULAR STRAND THEORY

 

Transcellular strand theory was proposed by Thiane et al. Al.   According to these people, certain protoplasmic strands are found within the sieve tube which traverses the sieve plats.  Organic solutes are assumed to move along these strands.  But such defined transcellular strands have never been observed by others.  But, recent electron microscopic studies reveal the presence of a large number of large numbers of P-proteins which run parallel to each other, across the sieve plates.  Visualization of such strands by these people without electron microscopic studies is highly creditable.  Full credit should be given to their concept of transcellular strands as means of translocation of organic solutes.

 

CONTRACTILE PROTEIN THEORY

 

Observations by Tension and Williams about the presence of a network of micro fibrils in sieve tubes and the involvement of such contractile proteins in the transport of solutes, though reported other, plant physiologists were skeptical about them.  It was very unfortunate for it is now known that such contractile P-proteins with their associated microtrabacular network are found within sieve tubes, is a fact.  And they are mainly responsible for the translocation.

 

Munich's "Pressure Flow" model (1927):

 

Mass or pressure flow hypothesis proposed by Munch has enjoyed wider acceptance.  The general agreement with this view is due to certain experimental evidences.  A membranous bag a filled with a lot of sucrose or other organic compounds and another membranous bag B-filled with just water are interconnected with a narrow bent glass tube.

 

 The bags are immersed in two interconnected troughs filled with water.   As soon as the bag A is immersed in water, water from the trough rushes into the membranous bag A and push the liquid from A towards B for bag A is filled with water only. 

 

 

Munich's model best describes the movement in the phloem. His model suggests that there is a turgor-pressure gradient that drives the directional mass flow of the solutes and water through sieve tubes of the phloem.

 

http://www2.mcdaniel.edu/

 

 This can be demonstrated with an osmometer permeable only to water filled with a high concentration of solutes in one arm as shown below. When the osmometer is put in distilled water, the water potential is less than that of surrounding water in the one arm, and water will enter by osmosis which the generates a turgor pressure, The solutes are carried by bulk flow when the water moves in. Eventually this process will stop when the pressure throughout equalizes. http://www2.mcdaniel.edu/